1. Field of the Invention
The present invention relates to the method for treatment of fly ash which is useful for production of concretes, mortars, and other mixtures comprising cement and fly ash. Fly ash manufactured according to the present method significantly improves performance of the concrete and provides higher level of replacement of standard Portland cement, which leads to significant economical and environmental benefits.
2. Description of the Related Art
Fly ash is a by-product of a coal burning power plant and is produced worldwide in large quantities each year.
Fly ash usually contains about 85% glassy, amorphous components. According to ASTM C 618 fly ash is classified in two classes, Class C and Class F. The Class F fly ash typically contains more than 70% by weight of silica, alumina, and ferric oxides, while Class C typically contains between 70% and 50%. Class F is produced as a by-product of the combustion of bituminous coal. Class C fly ash has a higher calcium content and is produced as a by-product of the combustion of sub-bituminous coal.
In 1988, approximately 84 million tons of coal ash were produced in the U.S. in the form of fly ash (ca 60.7%), bottom ash (ca 16.7%), boiler slag (5.9%), and flue gas desulphurization (16.7%), see, e.g., Tyson, 1990, xe2x80x9cCoal Combustion By-Product Utilizationxe2x80x9d, Seminar, Pittsburgh, 15 pp. Out of the approximately 50 million tons of fly ash generated annually, only about 10 percent is used in concrete, see, e.g., ACI Committee 226. 1987, xe2x80x9cUse of Fly Ash In Concrete, xe2x80x9d ACI 226.3R-87, ACI J. Proceedings 84:381-409), while the remaining portion is mostly deposited as waste in landfills.
Comprehensive research demonstrated that high volume fly ash concretes showed a higher long term strength development, a lower water and a gas permeability, a high chloride ion resistance, etc., in comparison with Portland cement concretes without fly ash.
At the same time, high volume fly ash concrete has significant drawbacks: a very long setting time and a very slow strength development during the period 0 to 28 days, which reduce the level of fly ash used for replacement of Portland cement to an average of 15-20%.
A number of efforts have been made towards improvement of the performance of high volume fly ash concretes; see, e.g., Malhotra, Concrete International J., Vol. 21, No. 5, May 1999, pp. 61-66. According to Malhotra, strength development of such concretes could be improved by significantly increasing the binder content (cement+microfiller) and significantly decreasing the amount of mixed water, but such an approach requires increased dosage of water reducing admixtures to keep an acceptable consistency of concrete mixtures, which sharply increases the cost of the concrete.
A number of methods related to grinding fly ash in order to improve its pozzolanic activity, which increases the amount of fly ash particles of the size of about 11 microns and by simultaneous introduction of calcium oxide, have been developed, see U.S. Pat. Nos. 6,038,987; 5,714,002; 5,714,003; 5,383,521, and 5,121,795. All mentioned known methods could not provide significant improvement of the fly ash performance as a concrete component, and they drastically increase the costs of pozzolan additive.
The present invention relates to a method for treatment of fly ash for preparation of mortars and concretes. In a first step fly ash is intensively blended with a highly-reactive and dry cement mixture, which mixture has been obtained by mixing Portland cement with a microfiller and possibly a water reducing agent and by grinding said mixture. In a second stage the so-obtained blend is interground in a vibratory milling device to achieve the fineness of the final product, with a retention on a 45 xcexcm sieve of less than 15 percent by weight.
According to a preferred embodiment, said milling device has a vibration cycle having an amplitude of from 2 to 30 mm and a frequency of vibration from 800 to 2000 rpm.